1. Field of the Invention
The invention relates to an apparatus for reproducing a digital recording.
2. Related Background Art
There is known a recording method whereby information such as an image signal is digitally recorded onto a magnetic tape by a helical scan system. A recording method of interleave recording under predetermined rules is used in order to improve a picture quality for a burst or a signal dropout in a normal reproducing mode, a reproduction disable region in a special reproducing mode, or the like (for example, JP-A-63-306504).
FIG. 1 is a plan view of a rotary drum of such a helical scan type digital recording and reproducing apparatus. FIG. 2 shows a development diagram of the rotary drum. Reference numeral 50 denotes a rotary drum; 52A, 52B, 54A and 54B, magnetic heads for recording or reproducing; and 56, a magnetic tape. Each of the heads 52A and 54A has a plus azimuth angle. Each of the heads 52B and 54B has a minus azimuth angle. Each of the heads 52A and 54A is arranged so as to be away from each of the heads 52B and 54B by only a small angle .theta. in the circumferential direction of the rotary drum 50. Each of the heads 52A and 52B is arranged so as to be away from each of the heads 54A and 54B by an angle of 180.degree. in the circumferential direction of the rotary drum 50. In the recording or reproducing mode, a pair of heads 52A and 52B and another pair of heads 54A and 54B are alternately used.
FIG. 3 shows a recording format of a magnetic tape in case of using eight tracks per one frame. A+ denotes tracks which are recorded or reproduced by the magnetic heads 52A and 54A. B- indicates tracks which are recorded or reproduced by the magnetic heads 52B and 54B. Image data of an even field (field #0) are recorded to the former half four tracks. Image data of an odd field (field #1) are recorded to the remaining four tracks.
FIG. 4 shows a data structure of one picture plane. In case of the NTSC system, one frame has 512 horizontal scanning lines and the image data are divided into synchronizing blocks on a line unit basis. On the CRT screen shown in FIG. 4, a solid line indicates a line of the even field and a broken line denotes a line of the odd field. In a line data block L.sub.n,m, n de notes a distinction (0 or 1) of the field and m indicates a line number. Each of the synchronizing blocks comprises: a sync code (sync) to match the synchronization timing; an identification code (ID) to identify each synchronizing block; image data of the line; and an error detection correction code P.
FIG. 5 is a block diagram showing a construction of a recording system of a conventional apparatus. FIG. 6 is a block diagram showing a construction of a reproducing system of the conventional apparatus.
In FIG. 5, an analog video signal to be recorded is supplied to an input terminal 10. An A/D converter 12 converts the analog video signal from the input terminal 10 into a digital signal. A coding circuit 14 compresses and encodes the output data from the A/D converter 12 by, for example, a DPCM coding method. The compression coded image data from the coding circuit 14 are written into a frame memory 16 and is also supplied to an error detection correction coding circuit 18. The coding circuit 18 generates the error detection correction code P to the compressed image data on a line unit basis and writes the resultant image data into the memory 16. The memory 16 ordinarily has a memory capacity of two frames. An address generation circuit 20 generates a write address and a read address of the memory 16.
A sync-ID addition circuit 22 adds a sync code (sync) and an ID for every line to the data read out from the memory 16, thereby forming a line data block as shown in FIG. 5. A modulation circuit 24 modulates an output of the sync-ID addition circuit 22 (for example, converts the output into an NRZI signal). An output signal of the modulation circuit 24 is magnetically recorded onto a magnetic tape 28 by a magnetic head 26. The magnetic head 26 corresponds to the magnetic heads 52A, 52B, 54A and 54B shown in FIGS. 1 and 2.
The reproducing system of FIG. 6 will now be described. The magnetic recording signal on the magnetic tape 28 is electromagnetically converted by the magnetic head 26. A demodulation circuit 30 demodulates the output of the magnetic head 26 and generates the digital signal of the line data block structure. A sync-ID separation circuit 32 separates the ID, image data, and error detection correction code P in accordance with the (sync) code sync from the line data block which is generated from the demodulation circuit 30. The reproduction image data and the error detection correction code P are supplied to a memory 34. The reproduction ID is supplied to an address generation circuit 36.
The address generation circuit 36 generates write addresses of the memory 34 in accordance with the reproduction ID. The memory 34 also generally has a memory capacity of two frames. An error detection correction circuit 38 detects and corrects errors in the image data stored in the memory 34 with reference to the error detection correction code P of the memory 34. The address generation circuit 36 generates a read address of the memory 34, thereby, reading out from memory 34 the image data of the frame whose error detecting correcting processes have been finished in the memory 34.
In case of reproducing the data at the same speed as that in the recording mode, the reproduction data are written into a first field memory area in the memory 34 in accordance with the ID of the line data block. In this instance, the data stored in another field memory area are read out. The memory area into which the data are written and the memory area from which the data are read out are changed for every field. FIG. 7 shows a memory space of a memory having a memory capacity of one frame. The ordinate indicates an address and the abscissa shows a time. Data are written in accordance with arrows shown by solid lines. The recorded data are read out in accordance with arrows shown by broken lines. Since the reading and writing operations of data are executed for different field memory areas, an outrun of the memory access does not occur.
A decoding circuit 40, shown in FIG. 6, executes a decoding process corresponding to the coding process in the coding circuit 14 to the data read out from the memory 34 and generates a digital image signal. A D/A converter 42 converts the digital output signal from the decoding circuit 40 into the analog signal. The analog signal is supplied from an output terminal 44 to a video monitor or the like.
In case of reproducing the data at the same speed as that in the recording mode, no problem occurs. In case of reproducing the data at a speed higher than that in the recording mode (hereinafter, such a reproduction is called a search reproduction), however, the magnetic head traces a plurality of tracks during each rotation. FIG. 8 shows a trace pattern for search reproduction. That is, data of different fields coexist in one frame period. Therefore, the address generation circuit 36 generates a write address existing in both of the odd field memory area and the even field memory area in the memory 34 in accordance with the reproduction ID. FIG. 9 shows the writing operation of the memory 34. The ordinate indicates an address and the abscissa shows a time. Since the data are reproduced by irregular addresses, the data are also irregularly written into the memory 34.
As mentioned above, since the writing operation is irregularly executed to the whole frame, there is no surplus time to read out the data. If the operator tries to forcedly read out data, the reading operation outruns the writing operation. To avoid such an outrun, hitherto, the frame memory having a memory capacity of at least two frames is used as a memory 34 and the memory area to write data and the memory area to read out the data are switched every frame.
When the frame memory having a large memory capacity of two frames is used, however, hardware size increases and costs also rise.
As will be clearly understood from a trace pattern shown in FIG. 8, not only data of a plurality of frames coexist in one frame period but also data which is not reproduced in one frame (or field) appears.
In case of the interlace system, the lines constructing one frame are alternately assigned to even fields and odd fields as shown in FIG. 10. Even if the data (for example, L.sub.0,1 and L.sub.1,1) of the lines which are neighboring on the screen could be reproduced, in case of an image which moves at a high speed, there is a case where a deviation between the times of those data is too large.
The line data block which has once been reproduced is not rewritten until the line data block of the same address in the memory 34 is reproduced. The rewriting period (hereinafter, called an updating period) is relatively long although it depends on the data shuffling or reproducing speed.
Because of the above reasons, a reproduced image becomes a remarkably unnatural image.